Thousands of biochemical processes are ongoing in the living body at any given time. Many of these endogenous aerobic processes naturally give rise, as by-products, to very highly reactive molecules. A large number of these reactive molecules are known generally as free radicals, which are defined as an atom or group of atoms with an unpaired electron. However, other non-free radical reactive species are also generated by these processes. The processes that produce the reactive entities may be enzymatic, such as those involved in phagocytosis, respiration, the cytochrome P-450 system and prostaglandin synthesis; or they may be non-enzymatic, such as the reaction of oxygen with organic compounds, or reactions initiated by ionizing radiation. These reactive molecules, if uncontrolled, may rapidly, and randomly, react with molecules in their vicinity, giving rise to toxic products that can interfere with the body's normal physiological processes. Considerable evidence exists that unchecked free radical reactions have some, if not major, involvement, in a number of disease states, for example, emphysema, inflammation, cancer, atherosclerosis and cataracts. Free radical reactions are also widely considered to have a major contributory effect on the natural aging process.
Among the most reactive of all regularly produced reactive species, and biologically among the most important, are those containing oxygen. These include, for example, partially reduced oxygen free radicals such as superoxide anion radicals, hydrogen peroxide and hydroxyl ions, as well as singlet oxygen. The latter, while not strictly speaking a free radical, as it technically possesses paired electrons, can be conveniently grouped with the oxygen-centered radicals, as its distorted electron configuration confers a high level of reactivity, and it is therefore potentially similarly toxic. These reactive oxygen species have been implicated in a number of reactions that can cause serious damage to cellular components: for example, oxidizing radicals can attack the bases and sugar molecules of DNA, altering the molecular structure and thereby interfering with biological functions. They may also interact with unsaturated fatty acids in cell membranes, causing lipid peroxidation, which results not only in alteration of the protein:lipid interaction of the membrane, but in the production of breakdown products which can exert a host of undesired effects, such as inhibition of DNA synthesis, adenyl cyclase and glucose-6-phosphate, increase in capillary permeability and inhibition of platelet aggregation.
Because molecular oxygen is virtually everywhere and it freely accepts electrons, these oxygen-centered radicals are probably the most common mediators of cellular free radical reactions. They are of course routinely produced as a result of aerobic metabolism. However, a very significant amount is generated as a result of photochemical reactions. Any organic or inorganic compound will absorb some UV radiation, and the absorbed energy will promote chemical reactions. There are a variety of recognized mechanisms by which light can cause the generation of oxygen-centered radicals; regardless of the mechanism, however, it is clear that the interaction of sunlight with organic or inorganic substrates on exposed skin can result in one or more reactive oxygen species being produced on the skin. It has been recognized in recent years that the presence of oxygen radicals on the skin is probably responsible for a number of the undesirable effects of prolonged exposure to the sun. For example, the aging phenomenon generally observed throughout the body is frequently observed prematurely on the skin as a result of photoaging, which accelerates the process of deterioration of elastin and collagen, among other effects. There is also an increased risk of skin cancer of all types.
The body has a number of defenses that can, under normal circumstances, to a large extent keep the potential damage resulting from these reactions in check. One of the most important of the naturally occurring defense mechanisms is the tripeptide glutathione comprising glutamic acid, cysteine, and glycine residues, and which is found in most cell types in the body. This compound has very significant free radical scavenging properties, and is believed to play a significant role in protecting cells against the cytotoxic effects of ionizing radiation, heat, certain chemicals, and significantly, solar UV radiation (Tyrell et al., Photochem. Photobiol. 47: 405-412, 1988) The mechanism by which glutathione protects cells against oxidative attack is complex, and involves a number of additional biochemical players; however, it is well established that a naturally occurring pathway involving glutathione and several glutathione-associated enzymes is capable of reducing a wide variety of organic hydroperoxides, thereby preventing substantial cellular oxidative damage. Indeed, it has also been shown that depletion or elimination of cellular glutathione can result in cellular sensitization to radiation, oxidative stress, decreased synthesis of leukotrienes and prostaglandins, inhibition of thermotolerance, decrease lymphocyte response to mitogens, and increased response to teratogens (Dolphin et al., eds., Glutathione: Chemical, Biochemical and Medical Aspects, Part A Series, Coenzymes and Cofactors. John Wiley and Sons, NY, 1989; Meister, J. Biol. Chem. 263: 205-217, 1988; Meister, Science 200: 471-477, 1985).
Clearly, a substantial intracellular supply of glutathione is critical to protect cells from the daily oxidative stress to which they are subjected, and given the broad array of exogenous stimuli which tax this system, it is expected that naturally occurring supply will be routinely depleted. While true in all areas of the body, this is particularly important in the skin, which is so greatly exposed to the damaging effects of radiation, particularly UV radiation. It is, therefore, highly desirable to determine a means of enhancing the generation of glutathione in cells, so as to maintain or replenish cellular levels which can readily respond to daily environmental insults. While a logical approach would seem to be to provide cells with an exogenous source of glutathione, the compound is not transported into the cells and therefore does not result in an intracellular accumulation of glutathione. Thus, there continues to be a need for finding alternate sources of glutathione enhancement. The present invention fill such a need.